This invention relates to a data communication apparatus and, more particularly, to a data communication apparatus as typified by a facsimile apparatus for sending and receiving images.
In many recent facsimile machines, the telephone numbers of facsimile machines belonging to other parties with which communication is frequent are registered beforehand in a memory backed up by the machine itself. Accordingly, rather than entering a telephone number when a transmission is to be made, the machine of the other party is called merely by pressing a minimal number of keys, which is followed by transfer processing.
In a case where data such as the telephone numbers of other parties are thus preserved in a back-up memory, usually a protective circuit of some kind is provided to protect the internally stored data. Ordinarily, a back-up memory requires not only processing for registering data but also a circuit which forbids the output of a memory-write signal.
A problem encountered in the prior art is that when a failure or malfunction develops in the circuitry including the back-up memory, the content of the back-up memory is lost when replacing circuit components or removing the battery in order to check the circuit.